Sunday, February 17, 2008

DNA Pollution Spawning Killer Microbes

Rogue genetic snippets spread antibiotic resistance all over the environment.

Over the past 50 years, virtually every known kind of disease-causing bacterium has acquired genes to survive some or all of the drugs that once proved effective against it. Analysis of a strain of vancomycin-resistant enterococcus, a potentially lethal bug that has invaded many hospitals, reveals that more than one-quarter of its genome—including virtually all its antibiotic-thwarting genes—is made up of foreign DNA. One of the newest banes of U.S. medical centers, a supervirulent and multidrug-resistant strain of Acinetobacter baumannii, likewise appears to have picked up most of its resistance in gene swaps with other species.

The antibiotic-drenched environment of commercial livestock operations is prime ground for such transfer. “You’ve got the genes encoding for resistance in the soil beneath these operations,” he says, “and we know that the majority of the antibiotics animals consume get excreted intact.” In other words, the antibiotics fuel the rise of resistant bacteria both in the animals’ guts and in the dirt beneath their hooves, with ample opportunity for cross-contamination.

These animal operations are real hot spots. They’re glowing red in the concentrations and intensity of these genes.

An even more direct conduit into the environment may be the common practice of irrigating fields with wastewater from livestock lagoons. About three years ago, David Graham, a University of Kansas environmental engineer, was puzzled in the fall by a dramatic spike in resistance genes in a pond on a Kansas feedlot he was studying. “We didn’t know what was going on until I talked with a large-animal researcher,” he recalls. At the end of the summer, feedlots receive newly weaned calves from outlying ranches. To prevent the young animals from importing infections, the feedlot operators were giving them five-day “shock doses” of antibiotics. “Their attitude had been, cows are big animals, they’re pretty tough, so you give them 10 times what they need,” Graham says.

The operators cut back on the drugs when Graham showed them that they were coating the next season’s alfalfa crop with highly drug-resistant bacteria. “Essentially, they were feeding resistance genes back to their animals,” Graham says. “Once they realized that, they started being much more conscious. They still used antibiotics, but more discriminately.”

While livestock operations are an obvious source of antibiotic resistance, humans also take a lot of antibiotics—and their waste is another contamination stream. Bacteria make up about one-third of the solid matter in human stool, and Scott Weber, of the State University of New York at Buffalo, studies what happens to the antibiotic resistance genes our nation flushes down its toilets.

Weber is now investigating how fertilizer derived from human sewage may contribute to the spread of antibiotic-resistant genes. “We’ve done a good job designing our treatment plants to reduce conventional contaminants,” he says. “Unfortunately, no one has been thinking of DNA as a contaminant.” In fact, sewage treatment methods used at the country’s 18,000-odd wastewater plants could actually affect the resistance genes that enter their systems.

Consumers may contribute to the problem of DNA pollution whenever they use antibacterial soaps and cleaning products. These products contain the antibiotic-like chemicals triclosan and triclocarban and send some 2 million to 20 million pounds of the compounds into the sewage stream each year. Triclosan and triclocarban have been shown in the lab to promote resistance to medically important antibiotics. Worse, the compounds do not break down as readily as do traditional antibiotics. Rolf Halden, cofounder of the Center for Water and Health at Johns Hopkins University, has shown that triclosan and triclocarban show up in many waterways that receive treated wastewater—more than half of the nation’s rivers and streams. He has found even greater levels of these two chemicals in sewage sludge destined for reuse as crop fertilizer. According to his figures, a typical sewage treatment plant sends more than a ton of triclocarban and a slightly lesser amount of triclosan back into the environment each year.

For consumer antibacterial soaps the solution is simple, Halden says: “Eliminate them. There’s no reason to have these chemicals in consumer products.” Studies show that household products containing such anti­bacterials don’t prevent the spread of sickness any better than ordinary soap and water. “If there’s no benefit, then all we’re left with is the risk,” Halden says. He notes that many European retailers have already pulled these products from their shelves. “I think it’s only a matter of time before they are removed from U.S. shelves as well.”